One GNSS is the Global Positioning System (GPS), which was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model, and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
To overcome the errors of standalone GPS systems such as satellite clock error, and propagation delays that result when the signal travels through the ionosphere and troposphere, many applications of GPS have made use of data from multiple GPS receivers. Typically, in such applications, a reference receiver, located at a reference site having known coordinates, receives the GPS satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance between the two GPS receivers, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and at the remote location, the errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates.
The technique of differencing signals from two or more GPS receivers to improve accuracy is known as differential GPS (DGPS). It includes local DGPS systems that utilize a single reference receiver that delivers either range measurements or corrections to its range measurements to one or more remote receivers so that the remote receivers can correct its range measurements. If range measurements rather than differential correctors are utilized, the remote receiver must know the location of the reference receiver so that it may compute the correctors (or their equivalent) internally. For brevity, throughout this disclosure, we shall refer to the data sent by the reference in either approach as differential correctors even though technically, in some instances, is simply range measurements that are sent. DGPS also encompasses Wide Area Differential GPS (WADGPS) where differential correction terms are generated by combining data from multiple reference GPS receivers spread geographically over a region of intended coverage. In all forms of DGPS, however, the positions obtained by the end user's remote receiver are relative to the position(s) of the reference receiver(s). Thus, absolute accuracy of any DGPS system depends heavily on the accuracy at which the reference receiver locations were determined when installing or implementing the DGPS system.
In many applications involving GNSS/GPS relative accuracy is often all that is necessary or desired. In these cases, the reference location need not be extremely accurate relative to any one particular coordinate system. That is, it is not a question of determining so much exact position, but position relative to some starting point with a high degree of accuracy. For example, the primary need for swathing applications that guide farm vehicles applying pesticides, fertilizer, and the like is to be able to guide the vehicle so that, relative to an initial swath, the subsequent swaths are at a series of prescribed offsets from the original swath (or from each other). There is often no accuracy requirement on the initial swath, only that subsequent swaths be accurate relative to the initial swath.
Of course, with relative positioning, it is still necessary to have the position of the reference location. A matter simply addressed if relative accuracy is indeed all that is required. For first time operation in a new geographic area, the reference location may be determined as the position of the GPS receiver as computed from the ensemble of the non-differentially corrected GPS range measurements at some point prior to going into differential mode. For future use in the same area a new reference may be determined, or the location may be retrieved from computer memory (or other sources) after having returned to a mark for which this location was determined. The location also could have been determined in a past operation of relative DGPS positioning. Finally, of course, the location could be manually supplied based on external information, such as a survey.
Standard methods of supplying differential corrections to a GNSS (typically GPS) receiver have been available for many years. For example, RTCM GPS correctors are sent out from fixed reference stations maintained by the Coast Guard (or other governmental agencies for non-US systems) and are transmitted in the 300 KHz radio band of the radio spectrum. Transmissions of such signals propagate over a few hundred kilometers. Commercial operators have also supplied RTCM correctors via VHF and UHF radio links operating over several tens of kilometers.
More recently, other sources of differential corrections designed primarily for single frequency GNSS receivers (L1 only receivers) have arisen such as those from Satellite Based Augmentation Systems (SBAS), an example of which is the Wide Area Augmentation System (WAAS). For dual frequency L1/L2 receivers, commercial satellite based correctors are available such as those from Omnistar or John Deere. Local differential correctors sent by radio can be supplied for operation in high accuracy real time kinematic (RTK) mode. Subscriptions to such L1/L2 based differential services are often expensive as is the dual frequency receiver technology. Many applications, such as swath guidance for farming, commonly require vehicle navigation or guidance accuracy exceeding that provided by SBAS capable L1 receivers alone and thus must rely on these more expensive technologies.
Therefore, what is needed is an economical means to rapidly deploy a cost effective GNSS differential reference station readily configured to supply differential correctors over a local area, typically 10 km radius or less.